Laminated structural members

ABSTRACT

APPARATUS AND METHODS ARE DISCLOSED FOR VACUUM-BAG MANUFACTURE OF THERMOSETTING RESIN-IMPREGNATED FABRIC LAYERS INTO FORMS SUITABLE FOR STRUCTURAL MEMBER APPLICATIONS.

Aprfifi 2%,, 197K LY MAUS LAMINATED STRUCTURAL MEMBERS Filed Oct. 15,1968 INVENTOR.

Louis Muus ATTORNEY United States Patent O U.S. Cl. 156103 7 ClaimsABSTRACT OF THE DISCLOSURE Apparatus and methods are disclosed forvacuum-bag manufacture of thermosetting resin-impregnated fabric layersinto forms suitable for structural member applications.

SUMMARY OF THE INVENTION Layers of a fabric impregnated with uncured orpartially-cured thermosetting resin, such as multiple plies of an epoxy,phenolic, or polyimide resin pre-impregnated glass fiber cloth, are cutto the required shape and are assembled on a base form Within a zonethat is defined by an edge dam element for lamination. Such edge damelement generally defines the edge of the desired laminated assembly andhas a substantially corresponding thickness to minimize or prevent theflow of resin from the assembled layers during resin curing. A releaseelement consisting of a ply of porous release fabric is positioned overthe fabric layers, a compression element of expanded honeycomb corematerial or the like is placed in edge-contacting relation over therelease element, a bleeder element of at least one ply of porous bleederfabric is placed over the honeycomb core compression element, and thefabric layers and superimposed apparatus elements are then sealed withina conventional vacuum-bag membrane element in a normal manner.Lamination is accomplished by heating the assembled fabric layers toresin curing temperatures in a prescribed manner and by simultaneouslypressure cycling the atmosphere environment within the vacuum-bagmembrane element also in a prescribed manner, each until resin curinghas been completed. In some instances a resin pool element consisting ofan extra ply of resin-impregnated fabric is positioned between therelease element and the compression element to provide a reservoir ofresin during lamination processing. Curing may be accomplished by aheating element that contacts the base form in heat transferrelationship or by conventional oven apparatus.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a preferred form ofapparatus for practicing the instant invention;

FIG. 2 is a sectional view taken at line 2-2 of FIG. 1; and

FIG. 3 is a partial sectional View of an alternate apparatus embodiment.

DETAILED DESCRIPTION FIG. 1 illustrates a preferred embodiment ofapparatus for practicing the instant invention to manufacture improvedlaminated structural members. Each element of the FIG. 1 illustration isseparately described in the following subparagraphs by the applicabledrawing reference numeral, element nomenclature, details of compositionand form, and statement of function, respectively:

11; fabric layer assembly; comprised of the required number ofindiivdual layers of thermosetting resinimpregnated fabric for thelaminated structural member and typically consisting of multiple pliesof WOVI1 glass fabric pre-impregnated with an epoxy, phenolic, orpolyimide thermosetting resin; cured at elevated temperature to formdesired laminated structural member;

3,575,756 Patented Apr. 20, 1971 12; base form; usually a metal orreinforced plastic, depending on whether subsequent heating is by aheater element or by oven apparatus, with an upper surface contoured tocorrespond to the surface contour of the laminated structural member andcoated with a release agent; functions to support fabric layer assembly11 and to conduct heat to the assembly at the assembly lowermost layer;

13; edge dam; preferably of zinc chromate tape adhesively secured tobase form 12; functions to minimize or prevent resin flow from assembly11 during lamination processing;

14; release element; preferably a ply of conventional compliant andporous release fabric (polytetrafiuorethylene or silicone lyophobicagent-coated release cloth, for example) cut to cover, and positionedover, assembly 11; functions to assure separation of superimposedapparatus elements, together with the release element, from assembly 11after lamination into the laminated structural member;

15; compression element; preferably comprised of expanded honeycomb corematerial, as for example expanded aluminum honeycomb core material of 7cell size, 4 pounds per cubic foot density or nylon-impregnated paperhoneycomb core material of /8" cell size, 2.3 pounds per cubic footdensity, placed in edge contacting relation over release element 14;functions to compress assembly 11 at its upper layer along andthroughout the line elements of a honeycomb network using relativelyhigh edge pressures (eg. p.s.i. to 1500 p.s.i.) developed by cycling theapparatus interior environmental pressure to and from near-vacuumconditions during laminations processing and may also function as amulti-celled reservoir for free liquid resin under particular conditionsof laminating processing;

16; bleeder element; preferably one or more plies of conventional wovenbleeder fabric; assists in the removal and admission of gases from theinterior-most portions of apparatus 10 during lamination processing;

'17; vacuum 'bag membrane; of conventional flexible polymericcomposition in sheet formand of a size to completely cover elements 11and 13- through 16; functions to isolate the environment vacuumpressures established at the apparatus interior during laminationprocessing from standard atmospheric environments at the apparatusexterior and also to develop the compression forces that act on assembly11 through compression element 15;

18; seal; preferably zinc chromate tape adhesively secured to the uppersurface of base form 12 and also to the peripheral undersurface ofvacuum-bag membrane 17; functions to seal the interior of apparatus 10at the edges of membrane 17 from exterior standard atmospheric pressureduring the vacuum phase of lamination processing;

19; vacuum fitting; conventional flanged tube sealed in apparatus 10 atan opening in membrane 17; serves as part of the means that connects theinterior of apparatus 10 to vacuum and standard atmospheric pressurecondi tions during lamination processing;

20; control system generally; essentially comprised of programmer andvalve assembly means and cooperating vacuum and standard atmosphericpressure sources; cycles the pressure condition at the interior ofapparatus 10 alternately between a vacuum pressure condition and acomparatively higher pressure condition such as atmospheric pressure ina prescribed manner during lamination processing;

21; programmer; normally in the form of separate electrical switch meansoperated by a timing cam and synchronous motor arrangement on aconventional on/off basis; continuously produces one or the other of twoseparate control signal outputs that control the valving of either avacuum or standard atmospheric pressure condition to the interior ofapparatus '10, each for preselected periods of time as hereinafterdescribed, during lamination processing;

22; valve assembly; preferably a solenoid-operated valve controller asto either of two operating conditions by programmer 21; responds to onecontrol signal from programmer 21 to valve a vacuum pressure conditionto the interior of apparatus and responds to the other control signalfrom programmer 21 to valve a standard atmospheric pressure condition tothe interior of apparatus 10;

23; valve port; normally in the form of a standard threaded connectionto a cooperating rigid pressure-resistant line fitting; connects valveassembly 22 to a conventional vacuum pressure source (not shown);

24; valve port; normally a conventional valve connection open to theexisting environment; connects valve assembly 22 to a standardatmospheric pressure source;

25; valve port; normally in the form of a standard threaded connectionthat cooperates with a conventional pressure-resistant line connectionfitting; connects valve assembly 22 to that portion of apparatus 10 thatincludes vacuum-bag membrane 17;

26; valving element; normally a metal cylinder provided with a. throughalignment passageway and rotated between two alternate positions by thesolenoid of valve assembly 22; functions at one position to connectvalve port 23 to valve port 25 to establish a vacuum pressure conditionat the interior of apparatus 10 and at the other position to connectvalve port 24 to valve port 25 to establish a standard atmosphericpressure condition at the interior of apparatus 10, each duringlamination process- 27; pressure line; conventional non-collapsinghosing connected at one end to fitting 19 and at the other end to valveport 25; functions as a part of the means that connects the interior ofapparatus 10 to valve assembly 22;

28; heater assembly; comprised of a metallic body and suitable heatingelements; generates and transfers heat into base form 12 at a sufficienttemperature for curing the thermosetting resin in assembly :10 duringlamination processing;

29 (FIG. 2); resistance heating element; electrical resistance device ofconventional rod form inserted into the body of heater assembly 28 andconnected to an elec trical power supply; functions to convertelectrical energy received from the power supply to heat at a sufficientrate to accomplish curing of the thermosetting resin;

In addition to the foregoing elements, the apparatus of FIG. 1 maysometimes advantageously incorporate a resin pool element. Although notshown in the drawings, such resin pool element is in the form of one ormore additional fabric layers impregnated with the same thermosettingresin as that of assembly 11 and positioned in the apparatusintermediate release element 14 and compression element 15. Suchadditional element during lamination processing provides an excess ofthermosetting resin to assure an adequate quantity of that material andadditionally cushions the underside edges of compression element '15 ina manner whereby there is a minimum embossing of the upper surface ofthe resulting laminated structural member with the corresponding patternof a network of line elements. Cushioning may alternately beaccomplished by use of a layer of Wire screening at the underside ofcompression element 15.

As indicated above, the apparatus arrangement 10 of FIG. 1 utilizesheater assembly 28 for accomplishing resin curing during laminationprocessing. Other means for heating base form 12 to cure thethermosetting resin of assembly 11 may also be utilized in the practiceof the instant invention and one suitable alternate apparatusarrangement for this purpose is disclosed by FIG. 3. In the FIG. 3arrangement, which arrangement is intended for use with conventionaloven apparatus to accomplish resin curing, a heat sink means 31 ispositioned over vacuum-bag membrane 17 for the purpose of assuring atemperature differential across assembly 11 during laminationprocessing. In this regard it is important that resin curing beaccomplished during lamination processing in a directional manner fromthe lowermost fabric layer of assembly 11 to the uppermost fabric layer.In some instances it is possible to eliminate the requirement for heatsink 31 even though heating is accomplished by conventional ovenapparatus. In such instances, cycling room temperature air into theenvironment interior of vacuum-bag membrane 17 at a comparatively highfrequency as hereinafter explained will function to positively establishthe temperature differential that obtains the required resin curingdirectionality.

As previously indicated, it is important that lamination processing beaccomplished in two critical manners. First, it is necessary in thepractice of the instant invention that the thermosetting resin fractionof fabric layer assembly 11 be cured directionally from the region ofthe lowermost fabric layer to the region of the uppermost fabric layer.Such is essentially accomplished by heating the assembly to theprescribed resin curing temperature from adjacent such lowermost fabriclayer. Second, it is necessary in the practice of this invention thatthe environment located interiorly of vacuum-bag membrane 17 and baseform 12 and containing fabric layer assembly 11 be pressure-cycledduring the period of directional resin curing alternately between vacuumand standard atmospheric (or comparably elevated) pressure conditions.The resin curing temperatures and times-at-temperature utilized are thestandard cure temperatures and times for the thermosetting resinactually being laminated. Such are normally established and prescribedby the manufacturer or supplier of the resin system.

The time characteristics of the different pressure phases of theindividual cycles that are continuously repeated during laminationprocessing in accordance with this invention may be varied. Individualpressure-varied cycles comprised of a 15 minute period of vacuumpressure condition followed by a 3 minute period of standard atmosphericpressure have been utilized in some instances. Individual cyclescomprised of a 2 minute period of vacuum pressure condition followed bya /2 minute period of standard atmospheric pressure or even a higherfrequency of A minute period of vacuum pressure condition followed by aA minute period of standard at mospheric pressure, on the other hand,are also advantageous as when lamination processing is to beaccomplished in conventional oven means without employing a heat sinkelement such as 31 of FIG. 3.

The invention of this application has been utilized in connection withthe lamination of different structural members employing glassfabric-reinforced phenolic, epoxy, and polyimide resin systems. Specificexamples of such lamination processing, together with details of theimproved structural properties that have been obtained, are as follows:

Example I Thirteen plies of a Type 181 weave E-glass fiber fabric, eachpre-impregnated with a commercially-available vacuum-bag grade phenolicresin system having a recommended standard cure of 2 hours at F.followed by 1 hour at 200 F., 1 hour at 240 F., and 1 hour at 275 F.,were laminated in accordance with this invention using apparatussubstantially similar to that illustrated in FIG. 1. The apparatusenvironment containing the assembled fabric layers was pressure cycledthroughout lamination processing utilizing continuously repeatedindividual pressure cycles comprised of 15 minutes vacuum phase at about27" Hg vacuum pressure followed by 3 minutes elevated pressure phase atambient (1 atmosphere) pressure. The assembly was laminated using thestandard temperature-time cure and was afterwards removed from theapparatus and post-cured at 400 F. for

2 hours. The resulting laminated structural member exhibited an edgewisecompressive strength of 67,290 p.s.i., an edge modulus of 4.61 p.s.i.,and a density of 1.76 grams per cubic centimeter. Such properties aresignificantly increased over the edgewise compressive strength of 37,300p.s.i., edge modulus of 3.03 10- p.s.i., and density of 1.58 grams percubic centimeter obtained in an identically constructed panel laminatedusing the same type of vacuum-bag apparatus but using a conventionalsteady state vacuum pressure of 27" of Hg throughout the standard cure,such identically constructed panel serving as a standard for comparisonpurposes. The reference panel was also subjected to the standardpostcure of 400 F, for 2 hours following removal from the conventionalvacuum-bag apparatus and prior to testing.

Example II Thirteen plies of a Type 181 weave E-glass fiber fabric, eachpre-impregnated with a commercially-available vacuum-bag grade phenolicresin system having a recommended standard cure of 2 hours at 170 F.followed by 1 hour at 200 F., 1 hour at 240 F., and 1 hour at 275 F.,were laminated in accordance with this invention using apparatussubstantially similar to that illustrated in FIG. 1. The apparatusenvironment containing the assembled fabric layers was pressure cycledthroughout lamination processing utilizing continuously repeatedindividual pressure cycles comprised of 2 minutes vacuum phase at about27" Hg vacuum pressure followed by /2 minute elevated pressure phase atambient (1 atmosphere) pressure. The assembly was laminated using thestandard temperature-time cure and was afterwards removed from theapparatus and post-cured at 400 F. for 2 hours. The resulting laminatedstructural member exhibited an edgewise compressive strength of 63,530p.s.i., an edge modulus of 4.61 10 p.s.i., and a density of 1.79 gramsper cubic centimeter. Such properties are significantly increased overthe corresponding properties for the identically constructed referencepanel described in detail in connection with Example I above.

Example III Thirteen plies of a Type 181 weave E-glass fiber fabric,each pre-impregnated with a commercially-available vacuum-bag gradephenolic resin system having a recommended standard cure of 2 hours at170 F. followed by 1 hour at 200 F., 1 hour at 240 F., and 1 hour at 275F., were laminated in accordance with this invention using apparatuswhich was substantially similar to that illustrated in FIG. 1 and whichincorporated a resin pool element consisting of an additional ply of thesame resinimpregnated fabric positioned intermediate the release elementand compression element components. The apparatus environment containingthe assembled fabric layers was pressure cycled throughout laminationprocessing utilizing continuously repeated individual pressure cyclescomprised of 2 minutes vacuum phase at about 27" of Hg vacuum pressurefollowed by /2 minute elevated pressure phase at ambient (1 atmosphere)pressure. The lamination was accomplished using a cure temperature of170 F. for a period of 12 hours and was afterwards removed from theapparatus and post-cured at 400 F. for 2 hours. The resulting laminatedstructural member exhibited an edgewise compressive strength of 62,890p.s.i., an edge modulus of 4.57 10 p.s.i., and a density of 1.89 gramsper cubic centimeter. Such properties are significantly increased overthe edgewise compressive strength of 53,870 p.s.i., edge modulus of 3.6510 p.s.i., and density of 1.54 grams per cubic centimeter obtained in anidentically constructed panel laminated using the same type ofvacuum-bag apparatus but using a conventional steady state vacuumpressure of 27" of Hg throughout the 170 F., 12 hour standard curecycle. Such identically constructed panel was fabricated as a standardfor comparison purposes and was also subjected to the standard postcureof 400 F. for 2 hours following removal from the vacuum-bag apparatusand prior to testing.

Example IV Thirteen plies of a Type 181 Weave E-glass fiber fabric, eachpre-impregnated with a commercially-available vacuum-bag grade epoxyresin having a recommended standard cure of /2 hour at 180 F. followedby /2 hour at 225 F., 1 hour at 275 F., and 1 hour at 300 F., werelaminated in accordance with this invention using apparatussubstantially similar to that illustrated in FIG. 1. The apparatusenvironment containing the assembled fabric layers was pressure cycledthroughout lamination processing utilizing continuously repeatedindividual pressure cycles comprised of 2 minutes vacuum phase at about27" Hg vacuum pressure followed by /2 minute elevated pressure phase atambient (1 atmosphere) pressure. The assembly was laminated using atemperature-time cure of F. for 12 hours and was afterwards removed fromthe apparatus and post-cured at 300 F. for 2 hours. The resultinglaminated structural member exhibited an edgewise compressive strengthof 65,490 p.s.i., an edge modulus of 4.24 10 p.s.i., and a density of1.91 grams per cubic centimeter. Such properties are significantlyincreased over the edgewise compressive strength of 52,680 p.s.i., edgemodulus of 3.56 10- p.s.i., and density of 1.67 grams per cubiccentimeter obtained in an identically constructed panel usingconventional vacuum-bag and oven apparatus without a honeycomb corecompression element and using a steady state vacuum pressure of 27" Hgvacuum throughout the standard temperature-time cure. Also, theinter-laminar shear strength of the improved panel Was measured at 6,293p.s.i. and is significantly increased over the corresponding 5,763p.s.i. property for the identically constructed standard panel. Thereference panel was also subjected to the standard postcure of 300 F.for 2 hours following removal from the conventional vacuum-bag apparatusand prior to testing.

Example V Thirteen plies of a Type 481 Weave E-glass fiber fabric, eachpre-impregnated with a commercially-available vacuum-bag grade polyimideresin system having a recommended standard cure of room temperature to280 F. in 30 minutes followed by an increase to 310 F. in an additional180 minutes, a rapid increase to 350 F., and a hold at 350 F. for 90minutes, were laminated in accordance with this invention usingapparatus substantially similar to that illustrated in FIG. 1. Theapparatus environment containing the assembled fabric layers waspressure cycled throughout lamination processing using the continuouslyrepeated individual pressure cycles detailed above in connection withExamples II, III, and IV. The assembly was laminated using atemperature-time cure of 240 F. for 12 hours and was afterwards removedfrom the apparatus and post-cured at 750 F. for 2 hours. The resultinglaminated structural member exhibited an edgewise compressive strengthof 82,380 p.s.i., an edge modulus of 4.81 10 p.s.i., and a density of1.93 grams per cubic centimeter. Such properties are significantlyincreased because of this invention over the edgewise compressivestrength of 56,400 p.s.i., edge modulus of 3.49 l0 p.s.i. and density of1.49 grams per cubic centimeter obtained in a reference panelconstructed of 12 plies of the same fiber in a Type 181 weave withpolyimide resin system impregnation using a conventional vacuum-bag andoven apparatus arrangement without a honeycomb core compression elementand using a steady state vacuum pressure of 27" Hg vacuum throughout astandard cure cycle.

Example VI A 13-pIy panel similar in construction to the panel ofExample V was laminated in accordance with the instant invention butusing oven apparatus rather than a heater assembly. Also, roomtemperature air was used in connection with the vacuum/ pressure cyclingto maintain the required positive temperature differential rather thanachieving the same effect by supplementary cooling with means such asheat sink element 31. Curing was accomplished at 215 F. over a period of12 hours; vacuum/ pressure cycles of 2 minutes at approximately 27" Hgvacuum followed by /2 minute at atmospheric pressure were employedthroughout the period of curing. The resulting panel developed a densityof 2.01 grams per cubic centimeter and an edgewise compressive strengthof 68,000 p.s.i. to compare very favorably over the conventionallylaminated reference panel.

I claim:

1. A method of manufacturing a fabric layer assembly impregnated with anuncured thermosetting resin system into a laminated structural member,and comprising the steps of:

(a) containing a fabric layer assembly impregnated with an uncuredthermosetting resin system in an environment having a vacuum pressurecondition and simultaneously therewith contacting the upper surface ofsaid fabric layer assembly with a compression force having a networkdistribution and derived from a high level compression pressure actinguniformly along joined line-like elements of a corresponding networkthroughout the extent of said upper surface;

(b) then containing said fabric layer assembly in an environment havinga pressure condition substantially greater than said vacuum pressurecondition and simultaneously therewith contacting said fabric layerassembly upper surface with a compression force having said networkdistribution but derived from a compression pressure substantially lowerthan said high level compression pressure acting along saidcorresponding network joined line-like elements; and

(c) frequently and continuously repeating steps (a) and (b) alternatelyduring the heating of said fabric layer assembly directionally from asupport side of said assembly opposite said upper surface to an elevatedresin curing temperature until said uncured thermosetting resin systemis cured.

2. The invention defined by claim 1, wherein cycles each comprised ofsteps (a) and (b) in sequence are continuously repeated at a rate ofapproximately from 3 to complete cycles per hour in connection with step(c).

3. The invention defined by claim 1, wherein the ratio of the period ofstep (a) to the period of step (b) is in the range of from 1 to 6.

4. The invention defined by claim 1, wherein said step (a) compressionforce is derived from a high level compression pressure greater thanapproximately p.s.i. acting at and along said corresponding networkjoined line-like elements.

5. The invention defined by claim 1, wherein said step (b) compressionforce is derived from a compression pressure of approximately 1,500p.s.i. acting at and along said corresponding network joined line-likeelements.

6. The invention defined by claim 2, wherein the ratio of the period ofstep (a) to the period of step (b) is in the range of from 1 to 6.

7. The invention defined by claim 4, wherein said step (b) compressionforce is derived from a compression pressure of approximately 1500p.s.i. acting at and along said corresponding network joined line-likeelements.

References Cited UNITED STATES PATENTS 2,713,378 7/1955 Nadler et al.156286UX 2,805,974 9/1957 Brucker 156-286X FOREIGN PATENTS 1,011,74412/1965 Great Britain l56-286 CARL D. QUARFORTH, Primary Examiner H. E.BEHREND, Assistant Examiner US. Cl. X.R.

